1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) system, and more particularly to a channel estimation method and apparatus in an OFDM system.
2. Description of the Related Art
Conventional methods for performing channel estimation in an OFDM system include pilot signal-based estimation and use of data decoded in a decision directed scheme. Usually, when coherent demodulation is used in a communication system, a transmitting end transmits pilot signals for channel estimation, and a receiving end for performing the coherent demodulation performs channel estimation based on the received pilot signals.
In a conventional OFDM system, a scheme for arranging pilots on the frequency-time plane may be classified into such schemes as a comb-type pilot arrangement and a lattice-type pilot arrangement.
The comb-type pilot arrangement scheme is used in a system in which a training symbol carrying pilots over the entire frequency axis is transmitted at the head, and data symbols uniformly carrying pilots through specific sub-carriers follow the training symbol in a wireless Local Area Network (LAN) where transmission/reception is performed in units of bursts without considering the mobility of a receiver. In this comb-type pilot arrangement, a channel value estimated in the training symbol is usually used in its entirety during a corresponding burst interval, and comb-type pilots are used for frequency tracking.
In contrast, the lattice-type pilot arrangement scheme is used in a broadcasting system where transmission/reception operate continuously, and even reception under a high-speed mobile environment is considered. In this arrangement, pilot sub-carriers are sparsely arranged in a certain pattern on the frequency-time plane, and spacing between the pilot sub-carriers falls within a coherence time and a coherence bandwidth such that interpolation using estimated channel values is possible.
In this manner, an OFDM receiver can constantly estimate and compensate for time-varying channel responses even during mobile reception through the aforementioned comb-type and lattice-type pilot arrangements, and consequently can continue to stably receive data.
Reference will now be made to a two-dimensional interpolation method for estimating a channel value at a pilot sub-carrier from a channel estimate at another pilot sub-carrier, which has been estimated by any algorithm, with reference to the accompanying drawings. The following description will be given by exemplifying a Digital Multimedia Broadcasting-Terrestrial/Handheld (DVB-T/H) system among systems using an OFDM scheme for the convenience of explanation.
FIG. 1 illustrates a pilot arrangement in a conventional DVB-T/H system.
Referring to FIG. 1, the DVB-T/H system uses a combination of the comb-type and lattice-type pilot arrangement schemes. Here, pilots arranged according to the comb-type scheme are referred to as continual pilots, and pilots arranged according to the lattice-type scheme are referred to as scattered pilots. Also, in the pilot arrangement diagram of FIG. 1, the abscissa axis represents the frequency axis, and the ordinate axis represents the time axis.
In the DVB-T/H system in FIG. 1, interpolation is performed from channel values of the pilot sub-carriers arranged according to the lattice-type pilot arrangement scheme. An interpolation method includes a method of performing one-dimensional interpolation for each symbol in the direction of the frequency axis by using only pilot sub-carriers included in the same symbol and a method of performing two-dimensional interpolation at the sacrifice of many symbol delays.
The one-dimensional interpolation method does not cause delays, requires minimal memory capacity, and involves minimal calculations necessary for the interpolation. However, when delay spread is substantial, reception performance may be lowered because spacing between pilot sub-carriers is wide in the direction of the frequency axis. Therefore, the two-dimensional interpolation method is mainly used so as to solve this problem with the one-dimensional interpolation method.
In the conventional two-dimensional interpolation method, in order to minimize the effect of delay spread or Doppler spread, pilot spacing in the time axis is compared with that in the frequency axis, and linear interpolation begins with one axis where pilot spacing is narrower. Through the linear interpolation for the axis where pilot spacing is narrower, known values are obtained at positions between pilots in the other axis where pilot spacing is wider. Thus, since channel estimates at pilot sub-carriers, as well as the known values obtained from the linear interpolation, can be used together for interpolation to be applied to the other axis where pilot spacing is wider, the two-dimensional interpolation method can provide an effect of shortening an interpolation interval as compared to the initial pilot spacing.
FIG. 2 illustrates a two-dimensional channel interpolation method in a conventional DVB-T/H system. Here, similar to FIG. 1, the abscissa and ordinate axes represent the frequency and time axes, respectively, and symbol n denotes a currently received symbol. Thus, symbols n−1 to n−6 denote previously received symbols.
Referring to FIG. 2, in the DVB-T/H system, pilot spacing in the time axis is 4 symbols, and pilot spacing in the frequency axis is 12 sub-carriers. Thus, for channel estimates of sub-carriers, interpolation along the time axis with narrow pilot spacing is first performed as a first-time interpolation, with the result that channel estimates designated by circles 210 with left-oblique lines are obtained. If the interpolation along the time axis is repeated in the symbol n, all sub-carrier positions of the symbol n−3, corresponding to multiples of 3, are determined as known values designated by the circles 210 with left-oblique lines. Next, by performing a second-time interpolation along the frequency axis for the symbol n−3, remaining channel estimates designated by circles 220 with right-oblique lines can be calculated.
In the aforementioned conventional two-dimensional interpolation method, since it takes a delay of 3 symbols to obtain channel estimates of one complete symbol and prepare them for use in compensation, a memory capacity that can store all complete Fast Fourier Transform (FFT) outputs of previous 4 symbols including a current symbol is required.
Further, the aforementioned conventional interpolation method has a limitation on ensuring performance in a wireless environment where a terminal moves at high speed. To be specific, although the coherence time of a time-varying fading channel gradually decreases as the moving speed of a receiver increases, the pilot spacing in the time axis is fixed. When a terminal moves at low speed, there may be no problem in performing the time-axis interpolation at intervals of 4 symbols. However, when a terminal moves at high speed, an interpolation interval between symbols becomes larger than a coherence time, which causes interpolation errors. Further, the frequency-axis interpolation is subsequently performed using inaccurate intermediate values including the interpolation errors, and thus interpolation for remaining sub-carriers also results in non-reliable values. In the end, a problem of deterioration of the overall reception performance is caused.